S/MIME Version 3 Message Specification
RFC 2633

• Status
• IESG evaluation record
• IESG writeups
• Email expansions
• History

• Versions
• 08

Network Working Group                               B. Ramsdell, Editor
Request for Comments: 2633                                    Worldtalk
Category: Standards Track                                     June 1999

                 S/MIME Version 3 Message Specification

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Introduction

   S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
   consistent way to send and receive secure MIME data. Based on the
   popular Internet MIME standard, S/MIME provides the following
   cryptographic security services for electronic messaging
   applications:  authentication, message integrity and non-repudiation
   of origin (using digital signatures) and privacy and data security
   (using encryption).

   S/MIME can be used by traditional mail user agents (MUAs) to add
   cryptographic security services to mail that is sent, and to
   interpret cryptographic security services in mail that is received.
   However, S/MIME is not restricted to mail; it can be used with any
   transport mechanism that transports MIME data, such as HTTP. As such,
   S/MIME takes advantage of the object-based features of MIME and
   allows secure messages to be exchanged in mixed-transport systems.

   Further, S/MIME can be used in automated message transfer agents that
   use cryptographic security services that do not require any human
   intervention, such as the signing of software-generated documents and
   the encryption of FAX messages sent over the Internet.

1.1 Specification Overview

   This document describes a protocol for adding cryptographic signature
   and encryption services to MIME data. The MIME standard [MIME-SPEC]
   provides a general structure for the content type of Internet
   messages and allows extensions for new content type applications.

Ramsdell                    Standards Track                     [Page 1]
RFC 2633         S/MIME Version 3 Message Specification        June 1999

   This memo defines how to create a MIME body part that has been
   cryptographically enhanced according to CMS [CMS], which is derived
   from PKCS #7 [PKCS-7]. This memo also defines the application/pkcs7-
   mime MIME type that can be used to transport those body parts.

   This memo also discusses how to use the multipart/signed MIME type
   defined in [MIME-SECURE] to transport S/MIME signed messages. This
   memo also defines the application/pkcs7-signature MIME type, which is
   also used to transport S/MIME signed messages.

   In order to create S/MIME messages, an S/MIME agent has to follow
   specifications in this memo, as well as the specifications listed in
   the Cryptographic Message Syntax [CMS].

   Throughout this memo, there are requirements and recommendations made
   for how receiving agents handle incoming messages. There are separate
   requirements and recommendations for how sending agents create
   outgoing messages. In general, the best strategy is to "be liberal in
   what you receive and conservative in what you send". Most of the
   requirements are placed on the handling of incoming messages while
   the recommendations are mostly on the creation of outgoing messages.

   The separation for requirements on receiving agents and sending
   agents also derives from the likelihood that there will be S/MIME
   systems that involve software other than traditional Internet mail
   clients.  S/MIME can be used with any system that transports MIME
   data. An automated process that sends an encrypted message might not
   be able to receive an encrypted message at all, for example. Thus,
   the requirements and recommendations for the two types of agents are
   listed separately when appropriate.

1.2 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [MUSTSHOULD].

1.3 Definitions

   For the purposes of this memo, the following definitions apply.

   ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.

   BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.

   Certificate: A type that binds an entity's distinguished name to a
   public key with a digital signature.

Ramsdell                    Standards Track                     [Page 2]
RFC 2633         S/MIME Version 3 Message Specification        June 1999

   DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
   X.509.

   7-bit data: Text data with lines less than 998 characters long, where